The bacteriophage T4 is a powerful tool in the analysis of basic mechanisms of mutagenesis. The genetics of the rII region have been extensively used and inferences of particular mutagenic pathways can be made. With molecular cloning and DNA sequencing techniques we can now confirm certain inferred pathways. Cloned sequences will also aid in the biochemical analysis of certain T4 functions involved in fidelity and repair. The genome of bacteriophage T4 will be cloned by recombinant DNA techniques. The primary cloning vector will be M13. The cloned sequences will be pooled and probed for various regions of interest either by DNA:DNA hybridization or by their ability to complement or recombine with T4 phage defective for the desired activity. These regions will then be sequenced or used in biochemical analyses.